Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. In the measurement of blood flow characteristics, returning ultrasonic waves are compared to a frequency reference to determine the frequency shift imparted to the returning waves by flowing scatterers such as blood cells. This frequency, i.e., phase, shift translates into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
The change or shift in backscattered frequency increases when blood flows toward the transducer and decreases when blood flows away from the transducer. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity.
In standard color flow processing, a high pass filter known as a wall filter is applied to the data before a color flow estimate is made. The purpose of this filter is to remove signal components produced by tissue surrounding the blood flow of interest. If these signal components are not removed, the resulting velocity estimate will be a combination of the velocities from the blood flow and the surrounding tissue. The backscatter component from tissue is many times larger than that from blood, so the velocity estimate will most likely be more representative of the tissue, rather than the blood flow. In order to get the flow velocity, the tissue signal must be filtered out.
In the color flow mode of a conventional ultra-sound imaging system, an ultrasound transducer array is activated to transmit a series of multi-cycle (typically 4-8 cycles) tone bursts which are focused at the same transmit focal position with the same transmit characteristics. These tone bursts are fired at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. A series of transmit firings focused at the same transmit focal position are referred to as a "packet". Each transmit beam propagates through the object being scanned and is reflected by ultrasound scatterers such as blood cells. The echo or return signals are detected by the elements of the transducer array and then formed into a receive beam by a beamformer.
For example, the traditional color firing sequence is a series of firings (e.g., tone bursts) along the same position, which firings produce the respective receive signals: EQU F.sub.1 F.sub.2 F.sub.3 F.sub.4 . . . F.sub.M
where F.sub.1 is the receive signal for the i-th firing and M is the number of firings in a packet. These receive signals are loaded into a corner turner memory, and a high pass filter (wall filter) is applied to each down range position across firings, i.e., in "slow time". In the simplest case of a (1, -1) wall filter, each range point will be filtered to produce the respective difference signals: EQU (F.sub.1 -F.sub.2) (F.sub.2 -F.sub.3) (F.sub.3 -F.sub.4) . . . (F.sub.M-1 -F.sub.M)
and these differences are input to a color flow velocity estimator.
One of the primary advantages of Doppler ultrasound is that it can provide noninvasive and quantitative measurements of blood flow in vessels. Given the angle .theta. between the insonifying beam and the flow axis, the magnitude of the velocity vector can be determined by the standard Doppler equation: EQU .nu.=cf.sub.d /(2f.sub.0 cos .theta.)
where c is the speed of sound in blood, f.sub.0 is the transmit frequency and f.sub.d is the motion-induced Doppler frequency shift in the backscattered ultrasound signal. The Doppler effect results in a time variation in the phase of the backscattered signal.
U.S. patent application Ser. No. 09/065,212, filed on Apr. 23, 1998, discloses a method and an apparatus for imaging flow directly in B mode. A sequence of broadband pulses is transmitted to a transmit focal position, and the backscattered signals from this sequence are filtered to remove echoes from stationary or slow-moving reflectors along the transmit path. The resulting flow signals and a conventional B-mode vector are envelope detected and displayed, the flow image being superimposed on the tissue image. A B-mode flow image is formed by repeating the above procedure for multiple transmit focal positions across the region of interest. The filtering is performed in slow time (along transmit firings) and consists of a high-pass "wall" filter (e.g., an FIR filter) with B-mode image feed-through. The firing-to-firing filtering permits a longer FIR wall filter for better clutter suppression while increasing the cutoff frequency to a useful range. The wall filter increases the flow signal-to-clutter ratio. The resulting B-mode flow image has the advantages of low clutter from stationary or slow-moving tissue or vessel walls, high resolution, high frame rate and flow sensitivity in all directions. Flow sensitivity in the range direction is highest and arises from pulse-to-pulse radiofrequency (RF) decorrelation, while flow sensitivity in the cross-range direction is due to pulse-to-pulse amplitude decorrelation as a group of reflectors (e.g. blood or contrast agents) flows across the beam profile.
Wall filtering in flow estimation is intended to remove the large-amplitude echoes produced by stationary or very slow-moving tissue. Since these large-amplitude signals are conventionally not removed until after conversion of the analog echoes into digital acoustic data, very large-amplitude echoes can saturate the input to the analog-to-digital converters incorporated in the receive channels. Thus there is a need for a technique which enables the large-amplitude echoes corresponding to stationary or very slow-moving tissue to be removed prior to analog-to-digital conversion in the receiver.